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Friday, 26 September 2014

Weathered and abraded rock surfaces at Rhosyfelin







These are some of the rock surfaces currently exposed at Rhosyfelin within the area of the 2011-2014 dig.  They are all quite heavily weathered, and some are so deeply weathered that you can scrape the surface away with a sharp tool and get a soft grey powder from a deep gouge in the surface.  That speaks to me of long-continued exposure to cosmic bombardment, sunshine, wind and rain over many thousands of years.

More to the point, these surfaces (some on exposed bedrock outcrops, and some on detached slabs) are heavily abraded.  By which processes of erosion?  Well, the only ones that make sense here are glacial action and abrasion by torrents of meltwater during the deglaciation phase. This of course is supported by the abundance of large rounded boulders and stones exposed in the lowest part of the dig, under the present-day grassy valley floor.  There is no way that you could find these abraded surfaces in these positions if this was a quarry.  If the rock face was a man-made feature, you would expect all of this material to be jagged, angular and sharp-edged, like the material you find upslope on some of the more recent rock-falls:


......... or maybe the quarry owners employed apprentice quarrymen to go around polishing rock surfaces and rounding off sharp edges in case anybody might get hurt by them?

5 comments:

Dave Maynard said...

Have the words 'Processing Floor' appeared anywhere?

It is was a quarry, then this lower area is the location where large stones would be reduced in size to manageable portions ready for their trip elsewhere.

Dave

BRIAN JOHN said...

I've not heard any mention of a "processing floor" -- although I bet the archeos would dearly love to find one!! Maybe that's why they have extended the dig out onto the valley floor -- and all they have found is more till and a mixture of eroded slabs and boulders mixed in with torrential fluvio-glacial gravels and stones.

There has been some mention of "surfaces" -- but from what I have seen these are artifices, like a lot of other things -- coinciding with the top of the iron-pan or iron-stained sediments typical of this area.

Constantinos Ragazas said...

Brian,

Are the patterns of rounded and abrasive rock surfaces consistent with the likely direction of meltwater torrents?

Since you actually have seen these perhaps you can report on this.

Kostas

BRIAN JOHN said...

There is such a mess all over this site, following the excavations, that it's hard now to see what is in situ and what has been moved. But evidence of ice smoothing and scouring and meltwater flow on bedrock slabs and detached blocks are more or less where you would expect them -- much evidence of sharp edges rounded off around the tip of the bedrock spur, and many blocks embedded in the sediments also smoothed.

Constntinos Ragazas said...

Thanks for that Brian. So if the bedrock slabs show smoothing and scouring consistent with meltwater flow and the rockface also shows similar smoothing and scouring consistent with meltwater flow, couldn't we reasonably conclude there were meltwater flow? And so the Crag would have therefore been engulfed in water?

Interestingly, the “proto-orthostat” in front of the NW rockface does not show any signs of similar smoothing and scouring. Quite the opposite in fact. A point I had made more than a year ago. Which would indicate to me that it came from higher up the NW rockface and that it naturally slid down that ramp of fallen megaliths by the rockface. And not far from where the “proto-orthostat” came to rest. And that this occurred sometime after the meltwater flow ceased to exist. And likely dislodged from the rockface in the winter by ice frost. And pushed out of its place when its base became too eroded to support it. And slid down a snow embankment formed by the fallen megaliths at the base of the rockface. The reason why it is so “prestine”.

How does the rockface look higher up where the 'ramp of megaliths' is located? Likely that is where the “proto-orthostat” originated.

Kostas